Our guest blogger is Bill McKibben, co-founder of 350.org and the author of EAARTH: Making A Life in a Tough New World.

We have created a new planet. Not entirely new. It looks more or less like the one we were born into; the same physical laws operate it. But the changes that have already happened are large enough that if you were visiting our planet in a spaceship, this place would look really different from the outside than it did just decades ago — call it “Eaarth.”

I wrote the preface to my new book EAARTH on a gorgeous spring afternoon in 2009, perched on the bank of a brook high along the spine of the Green Mountains, a mile or so from my home in the Vermont mountain town of Ripton. The creek burbles along, the picture of a placid mountain stream, but a few feet away there’s a scene of real violence a deep gash through the woods where a flood in the summer of 2008 ripped away many cubic feet of tree and rock and soil and drove it downstream through the center of the village. Before the afternoon was out, the only paved road into town had been demolished by the rushing water, a string of bridges lay in ruins, and the governor was trying to reach the area by helicopter.

Twenty-one years ago, in 1989, I wrote the first book for a general audience about global warming, which in those days we called the “greenhouse effect.” That book, The End of Nature, was mainly a philosophical argument. It was too early to see the practical effects of climate change but not too early to feel them; in the most widely excerpted passage of the book, I described walking down a different river, near my then-home sixty miles away, in New York’s Adirondack Mountains. Merely knowing that we’d begun to alter the climate meant that the water flowing in that creek had a different, lesser meaning. “Instead of a world where rain had an independent and mysterious existence, the rain had become a subset of human activity,” I wrote. “The rain bore a brand; it was a steer, not a deer.”

Now, that sadness has turned into a sharper-edged fear. Walking along this river today, you don’t need to imagine a damned thing — the evidence of destruction is all too obvious. Much more quickly than we would have guessed in the late 1980s, global warming has dramatically altered, among many other things, hydrological cycles. One of the key facts of the twenty-first century turns out to be that warm air holds more water vapor than cold: in arid areas this means increased evaporation and hence drought. And once that water is in the atmosphere, it will come down, which in moist areas like Vermont means increased deluge and flood.

In our Vermont town, in the summer of 2008, we had what may have been the two largest rainstorms in our history about six weeks apart. The second and worse storm, on the morning of August 6, dropped at least six inches of rain in three hours up on the steep slopes of the mountains. Those forests are mostly intact, with only light logging to disturb them but that was far too much water for the woods to absorb. One of my neighbors, Amy Sheldon, is a river researcher, and she was walking through the mountains with me one recent day, imagining the floods on that August morning. “You would have seen streams changing violently like that,” she said, snapping her fingers. “A matter of minutes.” A year later the signs persisted: streambeds gouged down to bedrock, culverts obliterated, groves of trees laid to jackstraws. . . .

Global warming is no longer a philosophical threat, no longer a future threat, no longer a threat at all. It’s our reality. We’ve changed the planet, changed it in large and fundamental ways. And these changes are far, far more evident in the toughest parts of the globe, where climate change is already wrecking thousands of lives daily. In July 2009, Oxfam released an epic report, “Suffering the Science,” which concluded that even if we now adapted “the smartest possible curbs” on carbon emissions, “the prospects are very bleak for hundreds of millions of people, most of them among the world’s poorest.”

And so EAARTH is, by necessity, less philosophical than its predecessor. We need now to understand the world we’ve created, and consider urgently how to live in it. We can’t simply keep stacking boulders against the change that’s coming on every front; we’ll need to figure out what parts of our lives and our ideologies we must abandon so that we can protect the core of our societies and civilizations. There’s nothing airy or speculative about this conversation; it’s got to be uncomfortable, staccato, direct.

Which doesn’t mean that the change we must make or the world on the other side will be without its comforts or beauties. Reality always comes with beauty, sometimes more than fantasy. But hope has to be real. It can’t be a hope that the scientists will turn out to be wrong, or that President Barack Obama can somehow fix everything. Obama can help but precisely to the degree he’s willing to embrace reality, to understand that we live on the world we live on, not the one we might wish for. Maturity is not the opposite of hope; it’s what makes hope possible.

From the Book EAARTH: Making a Life on a Tough New Planet by Bill McKibben. Copyright (c) 2010 by Bill McKibben. Reprinted by arrangement with Henry Holt and Company, LLC. All rights reserved.